Kinescope driver with kinescope current sensing circuit

ABSTRACT

A television receiver includes a kinescope and a current sensing transistor for conveying amplified video signals to the kinescope, and for providing at a sensing output terminal an output signal related to the magnitude of kinescope current conducted during given sensing intervals. A clamping circuit clamps the sensing output terminal during normal image intervals, and unclamps the sensing output terminal during the sensing intervals. The clamping circuit facilitates interfacing the sensing transistor with utilization circuits which process the sensed output signal, and assists to maintain a proper operating condition for the sensing transistor.

This invention concerns a video output display driver amplifier forsupplying high level video output signals to an image display devicesuch as a kinescope in a television receiver. In particular, thisinvention concerns a display driver stage associated with a sensingcircuit for providing a signal representative of the magnitude ofcurrent conducted by the kinescope during prescribed intervals.

Video signal processing and display systems such as television receiverscommonly include a video output display driver stage for supplying ahigh level video signal to an intensity control electrode, e.g., acathode electrode, of an image display device such as a kinescope.Television receivers sometimes employ an automatic black current (bias)control system or an automatic white current (drive) control system formaintaining desired kinescope operating current levels. Such controlsystems typically operate during image blanking intervals, at which timethe kinescope is caused to conduct a black image or a white imagerepresentative current. Such current is sensed by the control system,which generates a correction signal representing the difference betweenthe magnitude of the sensed representative current and a desired currentlevel. The correction signal is applied to video signal processingcircuits for reducing the difference.

Various techniques are known for sensing the magnitude of the black orwhite kinescope current. One often used approach employs a PNP emitterfollower current sensing transistor connected to the kinescope cathodesignal coupling path. Such sensing transistor couples video signals tothe kinescope via its base-to-emitter junction, and provides at acollector electrode a sensed current representative of the magnitude ofthe kinescope cathode current. The representative current from thecollector electrode of the sensing transistor is conveyed to the controlsystem and processed to develop a suitable correction signal.

In accordance with the principles of the present invention, there isdisclosed a kinescope current sensing arrangement wherein a currentsensing device is coupled to a kinescope for providing at an outputterminal a signal representative of the magnitude of the kinescopecurrent. A clamping circuit clamps the output terminal to a givenvoltage during normal image trace intervals. During prescribed kinescopecurrent sensing intervals, however, the clamping circuit is inoperativeand the sensed signal representative of the kinescope current isdeveloped at the output terminal. The clamping circuit advantageouslyfacilitates interfacing the current sensing device with control circuitsfor processing the sensed signal, and assists to maintain a properoperating condition for the current sensing device which, in a disclosedembodiment, also conveys video signals to the display device. Inaccordance with a feature of the invention, the clamping circuit isself-keyed between clamping and non-clamping states in response to therepresentative signal at the output terminal.

In the drawing:

FIG. 1 shows a circuit diagram of a kinescope driver stage withassociated kinescope current sensing and clamping apparatus inaccordance with the present invention; and

FIG. 2 depicts, in block diagram form, a portion of a color televisionreceiver incorporating the current sensing and clamping apparatus ofFIG. 1.

In FIG. 1, low level color image representative video signals r, g, bare provided by a source 10. The r, g and b color signals are coupled tosimilar kinescope driver stages. Only the red (r) color signal videodriver stage is shown in schematic circuit diagram form.

Red kinescope driver stage 15 comprises a driver amplifier including aninput common emitter amplifier transistor 20 arranged in a cascodeamplifier configuration with a common base amplifier transistor 21. Redcolor signal r is coupled to the base input of transistor 20 via acurrent determining resistor 22. Base bias for transistor 20 is providedby a resistor 24 in association with a source of negative DC voltage(-V). Base bias for transistor 21 is provided from a source of positiveDC voltage (+V) through a resistor 25. Resistor 25 in the base circuitof transistor 21 assists to stabilize transistor 21 against oscillation.

The output circuit of driver stage 15 includes a load resistor 27 in thecollector output circuit of transistor 21 and across which a high levelamplified video signal is developed, and opposite conductivity typeemitter follower transistors 30 and 31 with base inputs coupled to thecollector of transistor 21. A high level amplified video signal R isdeveloped at the emitter output of follower transistor 30 and is coupledto a cathode electrode of an image reproducing kinescope via a kinescopearc current limiting resistor 33. A resistor 34 in the collector circuitof transistor 31 also serves as a kinescope arc current limitingresistor. Degenerative feedback for driver stage 15 is provided byseries resistors 36 and 38, coupled from the emitter of transistor 31 tothe base of transistor 20.

A diode 39 connected between the emitters of transistors 30 and 31 asshown is normally reverse biased and therefore nonconductive by thevoltage difference across it equalling the sum of the two base-emittervoltage drops of transistors 30 and 31, but is forward biased andtherefore rendered conductive under certain conditions in response topositive-going transients at the emitter of transistor 30, correspondingto the output terminal of driver stage 15. The arrangement of transistor31 prevents the amplifier feedback loop including transistors 20, 21 and31 and resistors 36 and 38 from being disrupted, thereby preventingfeedback transients and signal ringing from occurring. Additionaldetails of the arrangement including transistors 30 and 31 and diode 39are found in my copending U.S. patent application Ser. No. 758,954titled "FEEDBACK DISPLAY DRIVER STAGE".

The emitter voltage of transistor 30 follows the voltage developedacross load resistor 27, and transistor 30 conducts the kinescopecathode current. Substantially all of the kinescope cathode currentflows as collector current of transistor 30, through a kinescope arccurrent limiting protection resistor 37a, to a clamping network 40.Transistor 30 acts as a current sensing device in conjunction withnetwork 40 as will be explained. Clamping network 40 in this example isself-keyed to exhibit clamping and non-clamping states in response tothe magnitude of the current conducted by transistor 30.

Clamping network 40 is common to all three driver stages of thereceiver, as will be seen subsequently in connection with FIG. 2, and iscoupled to the green and blue signal driver stages via protectionresistors 37b and 37c. Network 40 includes clamping transistors 41 and42 arranged in a Darlington configuration, and series voltage dividerresistors 43 and 44 which bias clamp transistors 41 and 42. A highfrequency bypass capacitor 46 filters signals in the collector circuitof transistor 30 in a manner to be described below. The seriescombination of a mode control switch 49 and a scaling resistor 48 iscoupled across resistors 43 and 44. A voltage related to the magnitudeof kinescope current is developed at a terminal A and, as will beexplained with reference to FIG. 2, the voltage at terminal A can beused in conjunction with a feedback control loop to maintain a desiredkinescope operating current condition which is otherwise subject todeterioration due to kinescope aging and temperature effects, forexample.

Assuming switch 49, the function of which will be explained below, isopen, the kinescope cathode current flowing in the collector oftransistor 30 is conducted to ground via resistors 43 and 44. When thiscurrent causes a voltage drop across resistor 44 to sufficiently forwardbias the base-emitter junctions of transistors 41 and 42, transistor 42will conduct in a linear region, and will clamp terminal A to a voltageVA according to the following expression, where V_(BE41) and V_(BE42)are the base-emitter junction voltage drops of transistors 41 and 42:

    VA=(V.sub.BE41 +V.sub.BE42) (R43+R44)/R44

During normal image intervals typically there are greater thanapproximately 25 microamperes of current conducted by transistor 30,which is sufficient to render transistors 41 and 42 conductive fordeveloping clamping voltage VA at terminal A. At other times, as will bediscussed, transistors 41 and 42 are rendered nonconductive wherebyclamping action is inhibited and a (variable) voltage is developed atnode A as a function of the magnitude of the kinescope cathode current,for processing by succeeding control circuits.

Illustratively, the arrangement of FIG. 1 can be used in connection withdigital signal processing and control circuits in a color televisionreceiver employing digital signal processing techniques, as will be seenin FIG. 2. Such control circuits include an input analog-to-digitalconverter (ADC) for converting analog voltages developed at terminal Ato digital form for processing.

When the control circuits are to operate in an automatic kinescope blackcurrent (bias) control mode, wherein during image blanking intervals thekinescope conducts very small cathode currents on the order of a fewmicroamperes, approximating a kinescope black image condition, clamptransistors 41 and 42 are rendered nonconductive because such smallcurrents flowing through resistors 43 and 44 from the collector oftransistor 30 are unable to produce a large enough voltage drop acrossresistor 44 to forward bias transistors 41 and 42. Consequently terminalA exhibits voltage variations, as developed across resistors 43 and 44,related to the magnitude of kinescope black current. The voltagevariations are processed by the control circuits coupled to terminal Ato develop a correction signal, if necessary, to maintain a desiredlevel of kinescope black current conduction by feedback action. In thisoperating mode switch 49, e.g., a controlled electronic switch, ismaintained in an open position as shown in response to a timing signalVT developed by the control circuits.

When the control circuits are to operate in an automatic kinescope whitecurrent (drive) control mode wherein during image blanking intervals thekinescope conducts much larger currents representing a white imagecondition, switch 49 closes in response to timing signal VT, therebyshunting resistor 48 across resistors 43 and 44. The value of resistor48 is chosen relative to the combined values of resistors 43 and 44 sothat the larger current conducted via the collector of transistor 30divides between series resistors 43, 44 and resistor 48 such that themagnitude of current conducted by resistors 43 and 44 is insufficient toproduce a large enough voltage drop across resistor 44 to renderclamping transistors 43 and 44 conductive. Unclamped terminal Atherefore exhibits voltage variations related to the magnitude ofkinescope white current, which voltage variations are processed by thecontrol circuits to develop a correction signal as required. As usedherein, the expression "white current" refers to a high level ofindividual red, green or blue color image current, or to combined highlevel red, green and blue currents associated with a white image.

With the illustrated configuration of transistors 41 and 42 clampingvoltage VA is relatively low, approximately +2.0 volts. The clampingvoltage could be provided by a Zener diode rather than the disclosedarrangement of Darlington-connected transistors 41 and 42, but thedisclosed clamping arrangement is preferred because Zener diodes with avoltage rating less than about 4 volts usually do not exhibit apredictable Zener threshold voltage characteristic, i.e., the "knee"transition region of the Zener voltage-vs-current characteristic isusually not very well defined. In addition, the disclosed transistorclamp operates with better linearity than a Zener diode clamp andradiates less radio frequency interference (RFI).

The relatively low clamping voltage is compatible with the analog inputvoltage requirements of the analog-to-digital converter (ADC) at theinput of the control circuits which receive the sensed voltage atterminal A as will be explained in greater detail with respect to FIG.2. In this example the ADC is intended to process analog voltages offrom 0 volts to approximately +2.5 volts, and the clamping voltageassures that excessively high analog voltages are not presented to theADC during normal video signal intervals.

The relatively low clamping voltage also assists to prevent transistor30 from saturating, which is necessary since transistor 30 is intendedto operate in a linear region. To achieve this result and to maximizethe cathode current conduction capability of transistor 30, the clampingvoltage should be as low as possible to maintain a suitably low biasvoltage at the collector of transistor 30. On the other hand, the valueof arc current limiting resistor 37a should be large enough to provideadequate arc protection without compromising the objective ofmaintaining the collector bias voltage of transistor 30 as low aspossible. Operation of transistor 30 in a saturated state renderstransistor 30 ineffective for its intended purpose of properly conveyingvideo drive signals to the kinescope cathode, and for conveying accuraterepresentations of cathode current to clamping network 40 particularlyin the white current control mode when relatively high cathode currentlevels are sensed. In addition, undesirable radio frequency interference(RFI) can be generated by transistor 30 switching into and out ofsaturation. Also, when saturation occurs transistor base storage effectscan result in video image streaking due to the time required for atransistor to come out of a saturated state.

Thus clamping network 40 advantageously limits the voltage at terminal Ato a level tolerable by the analog-to-digital converter at the input ofthe control circuits coupled to terminal A, and protects theanalog-to-digital converter input from damage due to signal overdrive.Network 40 also provides a collector reference bias for transistor 30 toprevent transistor 30 from saturating on large negative-going signalamplitude transitions at its emitter electrode. The clamping voltagelevel is readily adjusted simply by tailoring the values of resistors 43and 44.

Capacitor 46 bypasses high frequency video signals to ground to preventtransistor 30 from saturating in response to such signals. Capacitor 46also serves to smooth out undesirable high frequency variations atterminal A to prevent potentially troublesome signal components such asnoise from interfering with the signal processing function of the inputanalog-to-digital converter of the control circuits, e.g., by smoothingthe current sensed during the settling time of the analog-to-digitalconverter.

The latter noise reducing effect is particularly desirable, for example,when the input ADC of the control circuits coupled to terminal A is ofthe relatively inexpensive and uncomplicated "iterative approximation"type ADC, compared to a "flash" type ADC. The operation of an iterativeADC, wherein successive approximations are made from the mostsignificant bit to the least significant bit, requires a relativelyconstant or slowly varying analog signal to be sampled during samplingintervals, uncontaminated by noise and similar effects.

The value of capacitor 46 should not be excessively large because acertain rate of current variation should be permitted at terminal A withrespect to kinescope cathode currents being sensed. If the value ofcapacitor 46 is too small, excessive voltage variations, particularlyhigh frequency video signal variations, will appear at terminal A,increasing the likelihood of transistor 30 saturating. The speed ofoperation of the clamp circuit itself is restricted by an RC low passfilter effect produced by the base capacitance of transistor 41 and theequivalent resistance of resistors 43 and 44.

FIG. 2 shows a portion of a color television receiver system employingdigital video signal processing techniques. The FIG. 2 system utilizeskinescope driver amplifiers and a clamping network as disclosed in FIG.1, wherein similar elements are identified by the same reference number.By way of example, the system of FIG. 2 includes a MAA 2100 VCU (VideoCodec Unit) corresponding to video signal source 10 of FIG. 1, a MAA2200 VPU (Video Processor Unit) 50, and a MAAA 2000 CCU (Central ControlUnit) 60. The latter three units are associated with a digitaltelevision signal processing system offered by ITT Corporation asdescribed in a technical bulletin titled "DIGIT 2000 VLSI DIGITAL TVSYSTEM" published by the Intermetall Semiconductors subsidiary of ITTCorporation.

In unit 10, a luminance signal and color difference signals in digitalform are respectively converted to analog form by means ofdigital-to-analog converters (DACs) 70 and 71. The analog luminancesignal (Y) and analog color difference signals r-y and b-y are combinedin a matrix amplifier 73 to produce r, g and b color imagerepresentative signals which are processed by preamplifiers 75, 76 and77, respectively, before being coupled to kinescope driver stages 15, 16and 17 of the type shown in FIG. 1. A network 78 in unit 10 includescircuits associated with the automatic white current and black currentcontrol functions.

The high level R, G and B color signals from driver stages 15, 16 and 17are coupled via respective current limiting resistors (i.e., resistor33) to cathode intensity control electrodes of a color kinescope 80.Currents conducted by the red, green and blue kinescope cathodes areconveyed to network 40 via resistors 37a-37c, for producing at terminalA a voltage representative of kinescope cathode current conducted duringmeasuring intervals, as discussed previously.

VPU unit 50 includes input terminals 15 and 16 coupled to terminal A.Through terminal 15 the VPU receives the analog signal from terminal Aand, via an internal multiplex switching network 51, the analog signalis supplied to an analog-to-digital-converter (ADC) 52. Terminal 16 isconnected to an internal switching device (corresponding to switch 49 inFIG. 1) which, in conjunction with scaling resistor 48, controls theimpedance and therefore the sensitivity at input terminal 15. Highsensitivity for black current measurement is obtained with resistor 48ungrounded by internal switch 49, and low sensitivity for white currentmeasurement is obtained with resistor 48 grounded by internal switch 49.

The digital signal from ADC 52 is coupled to an IM BUS INTERFACE unit 53which coacts with CCU unit 60 and provides signals to an output datamultiplex (MPX) unit 55. Multiplexed output signal data from unit 55 isconveyed to VCU unit 10, and particularly to control network 78. Controlnetwork 78 provides output signals for controlling the signal gain ofpreamplifiers 75, 76 and 77 to achieve a correct white currentcondition, and also provides output signals for controlling the DC biasof the preamplifiers to achieve a correct black current condition.

More specifically, during vertical image blanking intervals the three(red, green, blue) kinescope black currents subject to measurement andthe three white currents subject to measurement are developedsequentially, sensed, and coupled to VPU 50 via terminal 15. The sensedvalues are sequenced, digitized and coupled to IM Bus Interface 53 whichorganizes the data communication with CCU 60. After being processed byCCU 60, control signals are routed back to interface 53 and from thereto data multiplexer 55 which forwards the control signals to VCU 10.

What is claimed is:
 1. In a video signal processing system including animage reproducing device for displaying video information in response toa video signal applied thereto, apparatus comprising:a video outputdriver stage with a video signal input and a video signal output forproviding an amplified video signal; means for conveying said amplifiedvideo signal to said image reproducing display device, said conveyingmeans having a sensing output for providing thereat a sensed signalrepresentative of the current conducted by said image reproducingdisplay device; utilization means responsive to said sensed signal; andclamping means for selectively clamping said sensing output duringnormal image intervals, and for unclamping said sensing output duringintervals when said sensed signal representative of current conducted bysaid image reproducing display device is subject to processing by saidutilization means; wherein said clamping means comprises clampingtransistor means with an output first electrode coupled to said sensingoutput, a second electrode coupled to an operating potential, and aninput third electrode coupled to said sensing output, the conduction ofsaid clamping transistor means being controlled in accordance with themagnitude of said sensed signal as received by said third electrode; andsaid clamping transistor means is self-keyed to exhibit clamping andnon-clamping states in response to said sensed representative signal. 2.Apparatus according to claim 1, wherein:said video output stagecomprises a video amplifier with a video signal input and a video signaloutput for providing said amplified video signal; and said conveyingmeans comprises an active current conducting device with an input firstterminal for receiving said amplified video signal, an output secondterminal for conveying said amplified video signal to said imagereproducing display device, and a third terminal for providing saidsensed signal.
 3. Apparatus according to claim 2, whereinsaid activecurrent conducting device is a transistor with a base input forreceiving said amplified video signal, an emitter output for providingsaid amplified video signal to said image reproducing display device,and a collector output for providing said sensed signal.
 4. Apparatusaccording to claim 1, whereinsaid first and second electrodes define amain current conduction path of said clamping transistor means. 5.Apparatus according to claim 4, whereinsaid clamping means includesresistive means coupled to said sensing output for providing a voltagein accordance with the magnitude of said sensed signal; and said thirdelectrode of said clamping transistor means is coupled to said resistivemeans.
 6. Apparatus according to claim 1, and further comprisingfiltermeans for bypassing high frequency signal components at said sensingoutput.
 7. In a video signal processing system including an imagereproducing device for displaying video information in response to avideo signal applied thereto, apparatus comprising:a video output driverstage coupled to said image reproducing display device for providing anamplified video signal thereto, and having a sensing output forproviding thereat a sensed signal representative of the currentconducted by said image reproducing display device; control meansresponsive to said sensed signal for developing a control signal; meansfor coupling said control signal to said image reproducing displaydevice to maintain a desired conduction characteristic of said imagereproducing display device; and clamping means for selectively clampingsaid sensing output during normal image intervals, and for unclampingsaid sensing output during intervals when said control means operates tomonitor said sensed signal; wherein said clamping means comprisesclamping transistor means with an output first electrode coupled to saidsensing output, a second electrode coupled to an operating potential,and an input third electrode coupled to said sensing output, theconduction of said clamping transistor means being controlled inaccordance with the magnitude of said sensed signal as received by saidthird electrode; and said clamping transistor means is self-keyed toexhibit clamping and non-clamping states in response to said sensedsignal.
 8. Apparatus according to claim 7, whereinsaid control meansincludes digital signal processing circuits; and said control meansincludes an input analog-to-digital signal converter network.
 9. In avideo signal processing system including an image reproducing device fordisplaying video information in response to a video signal appliedthereto, apparatus comprising:a video amplifier with a video signalinput for receiving video signals, and a video signal output forproviding an amplified video signal; a signal coupling transistor withan input first electrode for receiving said amplified video signal fromsaid video amplifier, an output second electrode for providing a furtheramplified video signal to said image reproducing display device, and athird electrode for providing a sensed signal representative of themagnitude of the current conducted by said image reproducing displaydevice; utilization means responsive to said sensed signal; and clampingmeans for selectively clamping said third electrode of said couplingtransistor during normal image intervals, and for unclamping said thirdelectrode during interval when said sensed representative signal issubject to processing by said utilization means, said clamping meanscomprising clamping transistor means with an output first electrodecoupled to said third electrode of said signal coupling transistor, asecond electrode coupled to an operating potential, and an input thirdelectrode coupled to said third electrode of said signal couplingtransistor, the conduction of said clamping transistor means beingcontrolled in accordance with the magnitude of said sensed signal asreceived by said input third electrode of said clamping transistormeans.
 10. Apparatus according to claim 9, whereinsaid couplingtransistor is an emitter follower transistor with a base inputelectrode, an emitter output electrode, and a collector output electrodecorresponding to said third electrode.